Advances in polymerization and catalysts have produced polymer resins having improved physical and mechanical properties useful in a wide variety of products and applications. With the development of new catalysts, choices in polymerization processes, such as solution, slurry, high pressure, or gas phase, for producing a particular polymer have been greatly expanded. Advances in polymerization technology have also provided more efficient, highly productive, and economically enhanced processes.
Gas-phase polymerization processes are well known in the art. Such processes can be conducted, for example, by introducing the gaseous monomer or monomers into a stirred and/or fluidized bed of resin particles and catalyst. In fluidized-bed polymerization of olefins, the polymerization is conducted in a fluidized-bed reactor, wherein a bed of resin particles is maintained in a fluidized state by means of a gas stream including gaseous reaction monomer. The polymerization of olefins in a stirred-bed reactor differs from polymerization in a fluidized-bed reactor by the action of a mechanical stirrer within the reaction zone, which contributes to fluidization of the resin bed. As used herein, the term “gas-phase reactor” will include fluidized-bed and stirred-bed reactors.
The start-up of a gas-phase reactor generally uses a bed of pre-formed particles of polymer resin, known as a “seedbed.” After polymerization is initiated, the seedbed is sometimes referred to as a “reactor bed.” The reactor bed includes a bed of resin particles, catalyst(s), reactants and inert gases. This reaction mixture is maintained in a fluidized condition by the continuous upward flow of a fluidizing gas stream from the base of the reactor which includes recycle gas stream circulated from the top of the reactor, together with added make-up reactants and inert gases. A distributor plate is typically positioned in the lower portion of the reactor to help distribute the fluidizing gas to the reactor bed, and also to act as a support for the reactor bed when the supply of recycle gas is cut off. As fresh polymer resin is produced, polymer resin is withdrawn to substantially maintain the height of the reactor bed. Resin withdrawal is generally via one or more discharge outlets disposed in the lower portion of the reactor, near the distributor plate. The polymer resin withdrawn from the gas-phase reactor can be transferred into a product purge vessel. The polymer resin, for example, in the form of a polymer powder, may then be transferred out of the product purge vessel to downstream operations, which may include extrusion or packaging operations.
In normal operations, the polymer resin can intermittently be transferred from the product purge vessel to a seedbed container instead of to downstream operations. In some instances, a side stream of the polymer resin may be transferred to the seedbed container while continuing transfer to the downstream operations, thus allowing continued extrusion operations, for example. In the seedbed container, the polymer resin may be stored for subsequent use, for example, as a seedbed for reactor start-up. When needed, the polymer resin may be transferred from the seedbed container to the reactor. It is typically desired to have storage containers with stored polymer resins therein for each polymer resin that is to be made in the polymerization operations.
A conventional design of a seedbed storage system involves a closed-loop, pneumatic conveying system that does not allow for the polymer resin to be cooled during transfer to the seedbed container. Because the polymer resin is transferred at elevated temperatures (e.g., about 60° C. to about 110° C.), the resin may sinter if allowed to accumulate in the seedbed container without cooling and/or further circulation. The high solids to conveying fluid ratios in conventional conveying systems effect some cooling but do not achieve sufficient cooling to avoid sintering.
Accordingly, to avoid sintering, a cooling/recirculation step may be carried out after the transfer. The time before sintering occurs depends on resin properties, especially density, and can vary from a long time to almost no time. Thus, depending on the resin, there may not be adequate time to complete the transfer before sintering may occur. For example, there is a maximum transfer time of three hours for certain polymer resins before a cooling/recirculation step should be performed or the polymer resin in the seedbed container may sinter. Thus, the downstream operations, such as extrusion, typically must be shutdown and the full polymer resin stream typically must be transferred to the seedbed container so that the transfer to the seedbed container can be completed quickly enough to begin the cooling/recirculation step prior to sintering. Drawbacks to this approach include loss of operating continuity for the downstream operations and the risk of off-grade resin production.
Alternatively, the seedbed storage system may include two dilute-phase conveying systems. For example, the polymer resin may be transferred to the seedbed container using a conveying system while another conveying system re-circulates/cools the polymer product stored in the seedbed container. However, while this approach may allow transfer to the seedbed container while downstream operations, such as extrusion, are continued by transferring only a side stream of the polymer product, the expense and complexity associated with adding a second conveying system make this approach undesirable.
Accordingly, there exists a need for improved systems and processes for resin storage, such systems and processes capable of reducing the tendency for sintering while allowing for downstream operations to continue.